Galaxies are extremly large clusters of solar systems, gas, dust, and lot of other things. We classify galaxies based off shape and the 3 main types are sprial, irregular, and elliptical. Spiral galaxies have long arms around them, they have young stars and lots of gas. Elliptical galaxies have a round shape, old stars, and little gas. Irregulars have no defined shape, often have lots of gas and dust and young stars.